of transverse section; but a larger size
is preferable, as with large bearings the brasses do not wear so rapidly
and the straps are not so likely to be burst by the bearings becoming oval.
These sizes, as also those which immediately follow, suppose the pressure
on the piston to be 18 lbs. per circular inch.

348. _Q._--How is the strength of a cast iron gudgeon computed?

_A._--To find the proper size of a cast iron gudgeon adapted to sustain any
given weight:--multiply the weight in lbs. by the intended length of
bearing expressed in terms of the diameter; divide the product by 500, and
extract the square root of the quotient, which is the diameter in inches.

349. _Q._--What was Mr. Watt's rule for the strength of gudgeons?

_A._--Supposing the gudgeon to be square, then, to ascertain the thickness,
multiply the weight resting on the gudgeon by the distance between the
trunnions, and divide the product by 333. Extract the cube root of the
quotient, which is the thickness in inches.

350. _Q._--How do you find the proper strength for the cast iron beam of a
land engine?

_A._--If the force acting at the end of an engine beam be taken at 18 lbs.
per circular inch of the piston, then the force acting at the middle will
be 36 lbs. per circular inch of the piston, and the proper strength of the
beam at the centre will be found by the following rule:--divide the weight
in lbs. acting at the centre by 250, and multiply the quotient by the
distance between the extreme centres. To find the depth, the breadth being
given:--divide this product by the breadth in inches, and extract the
square root of the quotient, which is the depth. The depth of a land engine
beam at the ends is usually made one third of the depth at the centre (the
depth at the centre being equal to the diameter of the cylinder in the case
of low pressure engines), while the length is made equal to three times the
length of the stroke, and the mean thickness 1/108th of the length--the
width of the edge bead being about three times the thickness of the web. In
many modern engines the force acting at the end of the beam is more than 18
lbs. per circular inch of the piston, but the above rules are still
applicable by taking an imaginary cylinder with an area larger in the
proportion of the larger pressure.

351. _Q._--What was Mr. Watt's rule for the main beams of his engines?

_A._--Some of those beams were of wood and some of cast iron. The wood
beams w